1. Field of the Invention
This invention relates to a dry etching method and, more particularly, to a method for carrying out low-temperature anisotropic etching of a layer of a silicon oxide material at higher accuracy without employing a fluorocarbonaceous gas.
2. Description of Related Art
The technology of dry etching of a layer of a silicon oxide material with a high selectivity with respect to the layer of the silicon-based material is one of the etching processes extensively applied to manufacture of LSIs since the time when a 64K DRAM was developed. Typical of such dry etching is an etching of a silicon oxide interlayer insulating film, which is performed, with the use of a silicon substrate or a polycrystalline silicon layer as an etching base, for forming a contact on an impurity diffusion region formed in a semiconductor substrate or on a source/drain region of a pMOS transistor employed as a resistance load device of an SRAM.
Conventionally, for dry etching a layer of a silicon compound, such as silicon oxide, at a high selectivity with respect to the layer of a silicon-based material, CHF.sub.3, a CF.sub.4 /H.sub.2 mixed system, a CF.sub.4 /O.sub.2 mixed system or a C.sub.2 F.sub.6 /CHF.sub.3 mixed system, has been used as an etching gas. These etching gases are mainly composed of a fluorocarbonaceous gas having the ratio of the number of carbon atoms to that of fluorine atoms in a molecule (C/F ratio) of not less than 0.25. These gas systems are employed because (a) C contained in the fluorocarbonaceous gas forms a C--O bond on the surface of a silicon oxide layer to disrupt or weaken the Si--O bond, (b) an CF.sub.x.sup.+ ion above all a CF.sub.3.sup.+ ion, which is a main etchant for the layer of the silicon oxide layer, may thereby be yielded and (c) a state relatively rich in carbon may thereby be produced in a plasma, so that the oxygen in a silicon oxide is removed in the form of CO or CO.sub.2 ; on the other hand, a carbonaceous polymer is deposited on the surface of the layer of the silicon-based material, on account of the contribution of C, H and F contained in the gas system, thereby lowering the etching rate so as to give a high selectivity with respect to the layer of the silicon-based material.
It is noted that H.sub.2 and O.sub.2 as addition gases are used for controlling the selectivity, and are able to lower or raise the amount of yield of F*, respectively. In short, these gases have the effects of controlling the apparent C/F ratio of the etching reaction system.
In such an etching reaction system, attempts have been made to improve the selectivity with respect to the layer of the silicon-based material by increasing the supply of carbon. For example, the present inventor has proposed in Japanese Patent KOKAI Publication Hei-3-60032 (1991) a technology in which carbon is introduced by an ion implantation in the vicinity of a boundary surface between a silicon oxide layer and a silicon layer, and a technology in which a material composed of carbon is placed as a peripheral component of a sample wafer. With the former method by the ion implantation, carbon starts to be yielded when the etching reaches the vicinity of the boundary surface. However, as long as silicon oxide exists in the etched region, this carbon is removed out of the system as CO or CO.sub.2 without interfering with the etching of the layer of the silicon oxide-based material. However, if the silicon layer is exposed, carbon is deposited thereon in the form of a polymer to obstruct the etching of the silicon layer to improve the selectivity with respect to the silicon layer.
With the latter method of placing carbon-containing pieces of material as a peripheral component of the sample wafer, carbon is supplied to the system from time to time during etching by a sputtering effect to improve the selectivity by a mechanism similar to that described above. For example, such components as a cathode cover, wafer clamp or a susceptor may be constituted of such materials as silicon carbide, polycarbonate, Teflon or polyallylate.
With the above described conventional etching in which carbon is involved, an etching end point is usually determined on the basis of an abrupt decrease in intensity of a CO peak at 519 nm in the emission spectrum.
The above is an approach to establish a compatibility between anisotropy and selectivity through selection of an etching gas and selection of the material of peripheral components. In addition, in the field of dry etching of recent origin, a low temperature etching, that is an etching in which a sample wafer is maintained at a lower temperature of 0.degree. C. or lower during etching, as reported in Proceedings of Dry Process Symposium, 1988, pages 42 to 49, is re-attracting attention. Such low temperature etching has been proposed with a view to preventing shape defects, such as side etching, by inhibiting a radical reaction at the sidewall section, with an etching rate along the depth being maintained by ion-assist effects.
Meanwhile, flon compounds (chlorofluorocarbon gases) have been pointed out as leading to the destruction of an ozone layer surrounding the earth, and the prospect is high that manufacture and use thereof may be prohibited in the near future. Although the fluorocarbon gases are not encompassed within the category of specified flon compounds on which regulations have become more severe in recent years, the prospect is high that a ban may be placed on such fluorocarbon gas in the future. Under these circumstances, development of an etching gas as a substitute for the fluorocarbon gas and a technique of using such a substitute gas has become incumbent in the field of dry etching.
On the other hand, with further refinement of a design rule for a semiconductor device in the future, it may be feared that contamination by carbonaceous polymer particles derived from the fluorocarbon gases may bring out more grave consequences. Elimination of the fluorocarbon gases is also desirable in this consideration. Even if the fluorocarbonaceous gases can be eliminated, it is further feared that the above mentioned carbon-containing peripheral components may threaten to be a source of contamination, As a solution to this problem, it has been envisaged to constitute the wafer clamp or the cathode cover of quartz. However, this is tantamount to increasing the area to be etched at the time of etching of the layer of the silicon oxide-based material. The process may not be said to be practically useful in that the etching rate is significantly lowered under the loading effect.